This invention relates to a method and apparatus for use in scientific analysis of a sample. More specifically, it relates to the analysis of a sample under low temperature vacuum conditions.
Various scientific analysis techniques require, or give improved results, when the sample which is to be tested is cooled to a low temperature and placed under vacuum conditions. For example, various spectroscopic techniques involve analysis of a sample at low temperatures. One type of low temperature spectroscopic technique involves molecular luminescence. Such molecular luminescence is best observed when the samples are brought to liquid nitrogen temperature or below. Additionally, if a sample contains aromatic compounds dissolved in a suitable n-alkane solvent, extremely narrow-banded "quasi-linear" fluorescence spectra may result. This phenomenon, called the Shpol'skii effect, has become a valuable tool for the qualitative determination of polynuclear aromatic hydrocarbons.
The use of the Shpol'skii effect in the quantitative determination of polynuclear aromatic hydrocarbons has developed over the last few years such that the effect has now become a relatively useful tool in analytical spectrometry. The technique has been applied to the identification and quantitation of polynuclear aromatic hydrocarbons in environmental samples such as coal liquids, automobile exhaust, and air-borne particulates.
A major consideration encountered in the design of Shpol'skii spectrometric systems is the choice of the device used to cool the sample, this consideration also applying to other low temperature spectroscopic techniques. Three methods are commonly used for cooling in connection with Shpol'skii spectrometric systems. A liquid sample held in a quartz tube cell may be immersed in a Dewar flask filled with a coolant, such as liquid nitrogen or liquid helium. A second technique involves cooling a sample by conduction from a metallic rod immersed in a liquid coolant. A third technique involves cooling the sample while it is in contact with the cold stage of a Joule-Thomson refrigerator. Each of the methods has drawbacks. The handling of liquid nitrogen or liquid helium can be expensive, time consuming, and tedious, usually involving the manipulation of cumbersome Dewar flasks. On the other hand, Joule-Thomson refrigerators are usually large and bulky and often require elaborate vacuum systems and expensive gas compressors. As the refrigerators avoid the need for a liquid coolant, they have still been relatively attractive as sample cooling systems.
An important disadvantage with the Joule-Thomson refrigerators is the long cool down time associated with them. On the average, these refrigerators can cool a sample holding device from room temperature to 15.degree. K. in about one hour. Such a long cool down time makes the refrigerator arrangement impractical if each sample must be cooled separately. As a result, several designs allow deposition of more than one sample onto the cold stage at a time. When this is done, each sample must be moved individually onto the viewing position of the spectrometric system. At least two different techniques have been used for the deposit of more than one sample onto the cold stage.
A first approach is shown in U.S. Pat. No. 4,594,226 Reedy, this approach also being discussed in Analytical Chemistry, Volume 57, No. 8, July, 1985, "High-Resolution Gas Chromatography/Matrix Isolation Infrared Spectrometry" by Reedy et al., Pages 1602-1609. A sample is frozen onto a cold metal disk attached to the refrigerator cold stage. A double O-ring seal between the base of the refrigerator and the vacuum shroud allows the entire cryostat to rotate independently of the chamber. The sample, in the form of effluent from a gas chromatography column, is frozen onto the disk as it rotates. Three complete rotations of the cryostat may be achieved in order to provide six hours of sampling time. After this time, the cold stage must be heated to remove the collected samples and the entire sample collection process may be repeated.
A second approach for moving samples individually into the viewing system of a spectrometric system is disclosed in Applied Spectroscopy, Volume 40, No. 5, 1986, "A Multisurface Matrix-Isolation Apparatus" by Hauge et al. at Pages 588-595. This approach uses a sample holder which is rotated relative to the cold stage of a refrigerator. The sample holder is a hollow cylindrical metal block physically attached to the cold stage of the refrigerator by a flexible multi-leaf copper coil. Sixty (60) sample compartments are positioned around the cylinder in five rows. A particular row is chosen by moving the entire cryostat in the vertical direction. The sample within a particular row is chosen by turning the sample holder relative to the cold stage via a rotary vacuum feed-through. After the 60 samples are analyzed, the cold stage must be heated to remove them.
Although the prior techniques, such as the sample moving techniques discussed in detail above, are useful analytically, they still suffer from two significant disadvantages. First, both require that the entire cryostat be moved. As the cryostat is usually quite heavy and has several electrical connections and high pressure gas lines attached, this is often cumbersome. Accurate positioning of the cryostat requires expensive and elaborate mechanical devices. Secondly, both of the prior sample moving techniques significantly limit the sampling time between the heatings of the cold stage. When the sample holder is filled, and following the analysis of all of the samples, the apparatus is no longer available for analysis when the sample holder is cleaned by heating the cryostat. The "down time" during heating and recooling is significant.